Photochromism, which is expressed by various types of photochromic organic molecules, which are accepted into a host organic material, is a known phenomenon which is widely described in the literature.
Within a polymer matrix, the mobility of the polymer chains in the vicinity of the photochromic molecules defines the speed at which the photochromic properties of said molecules are expressed, insofar as the reversible change in colour of said molecules is based upon a reversible process of ring cleavage and isomerisation. Consequently, the process of darkening and fading of a polymer matrix, the host of the photochromic molecules, is dependent upon the free volume left, in said matrix, to said molecules and therefore, upon the flexibility of the polymer chains of said matrix.
From this point of view, the polyurethane or polyurethane-urea matrices are particularly interesting. The free volume within them can in fact easily be adjusted by a careful choice of the blocks assembled during the synthesis. Thus, the colouration and the return to the initial colour of a polyurethane or polyurethane-urea matrix can thus be optimised by an appropriate choice of the blocks incorporated during the synthesis of said matrix.
Furthermore, the use of photochromic coatings on non-photochromic matrices, instead of and in the place of matrices which are rendered photochromic throughout their whole mass, is opportune when said non-photochromic matrices do not offer a flexibility which is sufficient, a free volume which is sufficient, for the expression of the photochromism of molecules accepted within them. Thus, thermoplastic materials, such as polymethylmethacrylate or polycarbonates are not suitable as a host polymer of photochromic molecules insofar as the free volume within them and the flexibility of the chains constituting them are insufficient.
Patent application EP-A-0 294 056 describes the preparation of photochromic polyurethanes. Said polyurethanes are prepared, classically, from diisocyanate reagents and polyol reagents. The photochromic compound is added, prior to the reaction, with one of the reagents or in the mixture of reagents. The isocyanate groups being very reactive groups, the mixture of the reagents made up cannot be preserved. It must be made to react, immediately after its preparation. This constitutes a severe constraint with regard to the implementation of the process.
Additionally, if the isocyanate groups subsist within the polyurethane matrix prepared, they may generate, in the presence of moisture, (during the later use of said matrix), amines. Such amines are capable themselves of destabilising, even of destroying, certain photochromic compounds, which are present in the matrix (chromenes, in particular).
The U.S. Pat. No. 6,187,444 describes photochromic polyurethane coatings which are obtained by a synthesis process, which is exonerated by the constraint set forth above. The reactivity of the mixture of reagents—isocyanate/polyol—is in fact controlled insofar as blocked isocyanates are used. Their NCO functions are, classically, blocked with the aid of suitable organic compounds. When brought to a sufficient temperature—generally between 90 and 200° C.—the blocked isocyanates release the blocking compound and thus are isocyanates which are free to react with the polyols which are present, in order to generate the polyurethane sought after.
Unfortunately, as mentioned in this US patent, it is, in general, not possible to remove all the blocking compounds. Thus, an excess of blocked isocyanates must always be provided in order to ensure the presence of a given amount of free (unblocked) isocyanates. Such an excess of isocyanate is particularly unsuitable in a material which contains photochromic compounds. It has been seen above that the isocyanates can be hydrolysed into amines, and these amines are capable of destabilising, and of destroying, such photochromic compounds (more particular those of the chromene type).
Furthermore, during the synthesis of such coatings, notably of a significant thickness, it is not excluded to trap the blocking compounds. It is even possible, in certain cases, to observe the formation of bubbles on the surface of the coating.
The application WO-A-01 55 269 also describes photochromic polyurethane coatings.
One of the drawbacks of the classical polyurethanes is that they are quasi-biphasic with an ordered structure. Said polyurethanes in fact contain soft segments, which are entirely mobile, and which are present in the form of balls, and rigid oligourethane units, which are known as hard segments. The cohesion of these polyurethanes is mainly due to the hydrogen bonds which exist between the urethane groups, of high polarity, which are present in the hard segments. Consequently, it may be feared that the photochromic compounds which are localised in said hard segments suffer from a lack of mobility and therefore that the rates of darkening and of fading of the material in question be slowed down.
To this day, one has not any photochromic polyurethanes at ones disposal which are very efficient and which are easy to obtain.
The application WO-A-01 02 449 describes a photochromic coating, of poly(meth)acrylic type, which is obtained by copolymerisation of a mixture of (meth)acrylic monomers which contains photochromic molecules. Said mixture of monomers, which is not very viscous, does not enable coatings to be generated which are of a thickness sufficient in order to exhibit good photochromic properties, at a reasonable cost. In fact, in order to compensate for the low thickness of the coating, it is necessary that the photochromic compound be incorporated at a high concentration, and this sometimes poses a problem, in view of the solubility of said compound in said coating.